This invention relates to a color cathode ray tube and more particular, relates to a shadow mask covered with a porous layer.
Generally, a shadow mask type color cathode ray tube includes an electron gun emitting three electron beams, a phosphor screen, which is formed on an inner surface of a panel of an envelope and includes phosphor layers emitting red, green and blue light by bombardment of the electron beams, respectively, and a shadow mask provided in front of the phosphor screen with predetermined distance for selectively directing the electron beams so as to bombard the predetermined phosphor layers, respectively. In the cathode ray tube, reproduced images are observed through the panel. The shadow mask with a plurality of apertures is placed so as to have relationship that the apertures correspond to the phosphor layers, accurately.
In the cathode ray tube, the amount of effective electron beams passing through the apertures is 1/3 or less of the total amount of the electron beams emitted from the electron gun. The remainder of the electron beams are converted to thermal energy by bombarding the shadow mask. Thus, the shadow mask is heated up to the order of 80.degree. C. during the operation of a normal TV set. Moreover, in the special color cathode ray tube used in display such as in aircraft cockpits, the temperature of the shadow mask sometimes rises to about 200.degree. C.
Generally, such shadow mask is formed from thin plate with thickness of 0.1mm-0.3mm composed of so-called cold rolled steel with a thermal expansion coefficient as large as 1.2.times.10.sup.-5 /.degree. C. The shadow mask also has blackened film which is formed on the surface of the plate. The shadow mask is fixed to a mask frame in the envelope by securing a skirt portion of the shadow mask. The mask frame is formed from the same cold rolled steel with L-shaped cross-section of about 1 mm on which a blackened film is formed.
When the shadow mask is heated by the electron beams, the temperature of the peripheral portion of the shadow mask is lower than that of the central portion of the shadow mask since the peripheral portion is contacted with the mask frame with a large thermal capacity, so that the heat at the peripheral portion can be easily transmitted due to radiation and conduction. As the result, a temperature difference occurs between central portion and peripheral portion of the shadow mask, and a so-called doming phenomenon consequently occurs. When the doming phenomenon occurs, the distance between the shadow mask and the phosphor screen changes, and the color purity deteriorates due to disturbance to the accurate landing of the electron beams. This type of mislanding due to the doming phenomenom is remarkably during initial stage of the operation of the tube. Also, when an image including a high brightness portion is reproduced and the portion stops for a while, a local doming phenomenon occurs.
For settlement of the doming phenomenon by promotion of heat radiation from the central portion of the shadow mask, many proposals have been made. For example, it has been proposed in U.S. Pat. No. 2,826,538 that the shadow mask has a black layer consisting of graphite. Since the black layer acts as a good radiator, such temperature non-uniformity in the shadow mask can be prevented in some extent. However, since the adhesion of the black layer is reduce due to temperature changes during the heat treatment processing in the tube manufacturing process, and small pieces drop off by partial peeling due to the external vibration of the rube. The small pieces cause deterioration of the picture quality in the phosphor screen by blocking the apertures of the shadow mask. Further, the pieces cause deterioration in the withstand voltage characteristic by inducing sparking between the electrodes of the electron gun. Consequently, the quality of the color cathode ray tube is remarkably reduced due to the pieces.
Another proposal has been described in U.S. Pat. No. 4,716,333 in order to prevent the doming phenomenon by increasing the mechanical strength of the shadow mask. According to the patent, the shadow mask has a layer of lead borate glass which is bonded to the surface of the shadow mask and is formed by high-temperature heat treatment. The shadow mask can remarkably reduce the doming phenomenon due to the glass layer. However, since the glass layer contains lead with a large atomic number, it is difficult to reduce the elastic rebound of the electron beams which bombard the shadow mask.
Furthermore, it has been proposed in U.S. Pat. No. 4,734,615 that a shadow mask has a layer with a good heat dissipation property formed on the surface of the mask. The layer comprises a metal or metal oxide as a filler and an amorphous metal oxide as a binder.